STRESS INVERSION METHODS - ARE THEY BASED ON FAULTY ASSUMPTIONS

Stress inversion methods employed by structural geologists for estimat ing a regional stress tensor from populations of faults containing sli ckenlines rely on the basic assumption that slip on each fault plane o ccurs in the direction of maximum resolved regional shear stress. This premise ignores directional differences in fault compliance caused by fault shape, the Earth's surface or frictional anisotropy of the faul t itself. It is also assumed that the regional stress field is homogen eous in space and time. Thus, perturbations in the local stress field caused by such things as material heterogeneities near the fault and m echanical interaction with nearby faults are not considered. Regional stresses may exercise dominant control on the slip direction; however local factors may perturb this field. We show how differences in fault compliance and local stress perturbations can result in a measureable difference between the direction of resolved shear stress and the dir ection of fault slip. Numerical modeling of common fault geometries in an elastic half space provides a means for evaluating the magnitude o f this difference. We illustrate a few examples of geological circumst ances under which the inversion techniques should be reliable, and a f ew where errors related to violations of the basic assumptions exceed those inherent to the data gathering and inverse techniques.